The invention relates generally to magnetic sensors, and more particularly to a magnetic anomaly sensing system having a precision synchronized transceiver that directly measures magnetic field strength for improved detection and/or discrimination of targets.
The basic construction of a prior art eddy-current-based active magnetic anomaly sensor system includes a transmitter and a receiver. The transmitter induces anomalous magnetic induction fields in an electrically conductive or magnetic target located in the sensor detection space. The receiver detects/discriminates the anomalous magnetic induction fields propagating from the target. The transmitter typically consists of electronic circuitry that drives a time dependent electrical current through an induction coil to generate a time and vector distance dependent magnetic induction field. The induction coil can be driven by a continuous wave or pulsed signal. When the generated magnetic induction field interacts with a target, anomalous magnetic moments are induced with in the target which, in turn, cause anomalous magnetic fields to propagate from the target. The sensor system""s receiver typically consists of an induction or xe2x80x9csearch coilxe2x80x9d sensor coupled to signal amplification and processing circuitry to condition and process the magnetic fields detected by the search coil. Through Faraday induction, the search coil generates a voltage proportional to the time derivative of the target""s magnetic anomaly fields lying along the search coil""s axis. Such sensor systems have a variety of shortcomings.
For portable active sensor systems having a transmitter and receiver in close proximity to one another, the spatial variation between the actively induced magnetic anomaly field and its time derivative at the receiver decreases with the inverse 6-th power of target-to-receiver distance. Accordingly, to double the detection range of a sensor system, transmitter amplitude or receiver sensitivity must be increased by a large factor, i.e., a factor of 256 or 64. Also, relying on the time derivative of the magnetic anomaly field limits the sensor""s time discrimination capability, receiver bandwidth and low frequency sensitivity.
Another shortcoming of prior art active magnetic anomaly sensing systems is the interference generated by the transmitter at the receiver. Since the transmitter drive fields are many orders of magnitude larger than the target""s induced magnetic anomaly fields, the transmitted signal has a tendency to overwhelm or jam the reception of the much smaller magnetic anomaly fields. Even with specialized transmitter-receiver geometries, systems that use a continuous wave transmitter drive signal tend to lose much of a target""s transient response. To combat this problem, pulsed transmitters are used and operate on the theory that reception occurs when the transmitter is off. While this works to a certain degree, time constant or transient effects of a typical transmitter coil last for tens of microseconds. Unfortunately, it is in this time frame that the strongest target-signature-related magnetic anomaly field transients are generated by the target. Thus, even though the transmitter coil is deactivated, coil transients tend to jam reception of the strongest magnetic anomaly fields. This problem precludes the use of prior art active magnetic anomaly sensing systems in the detection of non-conductive plastic mines in a conductive media (e.g., seawater) since plastic mines have an extremely short transient response.
Still another shortcoming of prior art active magnetic anomaly sensor systems stems from the use of inductive search coils as the magnetic anomaly field sensing element. Specifically, this type of sensing element responds primarily to the time derivative of magnetic flux change components that are parallel to the coil""s axis. Therefore, the sensing element has limited spatial direction sensing capabilities for resolving the direction and magnitude of the three-dimensional vector components that comprise the magnetic anomaly field caused by the target. The lack of three-dimensional resolution limits the system""s target localization and classification capabilities.
Accordingly, it is an object of the present invention to provide an active magnetic anomaly sensing method and system for sensing magnetic anomalies associated with a target when the target is subjected to a magnetic induction field.
Still another object of the present invention is to provide an active magnetic anomaly sensing system capable of directly detecting induced magnetic anomaly fields associated with a target.
Yet another object of the present invention is to provide an active magnetic anomaly sensing system that minimizes interference between the transmitter and receiver portions thereof.
A further object of the present invention is to provide an active magnetic anomaly sensing system capable of resolving a target-generated magnetic anomaly field in three dimensions.
A still further object of the present invention is to provide an active magnetic anomaly sensing system capable of detection, localization and classification of electrically non-conductive targets such as plastic mines immersed in conductive media such as seawater.
Other objects and advantages of the present invention will become more obvious hereinafter in the specification and drawings.
In accordance with the present invention, an active magnetic anomaly sensing system includes a transmitter for transmitting a magnetic field towards a target such that the magnetic field induces magnetic moments in the target which cause a magnetic anomaly field to propagate from the target.
A first (magnetoresistive) sensor is positioned a distance D from the target for directly sensing magnetic field strength and producing a first output indicative thereof. A second (magnetoresistive) sensor is positioned a distance (D+d) from the target for directly sensing magnetic field strength and producing a second output indicative thereof. A controllable power supply is coupled to the transmitter for selectively activating and deactivating the transmitter. The first output and second output are produced when the transmitter is deactivated. The second output is subtracted from the first output to generate a differential output indicative of the magnetic anomaly field propagating from the target. Means and methods are provided to synchronize the response characteristics of the sensors with one another, and to synchronize the transmitter with the sensors so that deactivation of the transmitter results in a near instantaneous detection of magnetic field transients by the sensors.